Natural salt deposits are geological formations composed primarily of the mineral halite, the solid form of sodium chloride (NaCl). These deposits, often called rock salt, represent vast accumulations of evaporated brines buried within the Earth’s crust. Their defining characteristics include their chemical makeup, the unique processes by which they were created, and their resulting physical and structural properties.
Chemical and Mineralogical Composition
Natural salt deposits are defined by their high concentration of sodium chloride (NaCl). The mineral halite is composed of approximately 39% sodium and 61% chlorine by mass. While high purity is characteristic, natural deposits are rarely 100% pure, containing other minerals that reflect their depositional history.
The presence of associated minerals determines the commercial quality of the deposit. Common impurities include calcium sulfates (gypsum and anhydrite) and magnesium and potassium salts (sylvite and carnallite), often found in the upper layers. Insoluble residues, such as clay minerals, iron oxides, and quartz grains, can also be incorporated, often tinting the rock salt. Impurities can reach around 8% of the deposit, making purification necessary for many applications.
Geological Formation Processes
The characteristic process responsible for the formation of massive salt deposits is the evaporation of saline water, which concentrates the dissolved ions to the point of saturation. This process requires a specific set of geological conditions: an arid climate and a closed or semi-closed basin where the rate of water loss exceeds the rate of water input. These settings can be broadly categorized as marine or non-marine evaporites.
Marine evaporites form when large, restricted arms of ancient seas are periodically cut off from the open ocean and desiccate over long periods. Non-marine evaporites result from the evaporation of inland salt lakes or playas, where the source water chemistry can differ significantly from seawater. Regardless of the source, mineral precipitation follows a distinct, predictable sequence as the water volume decreases.
This predictable evaporite sequence is a defining feature of these deposits. As the brine concentrates, the least soluble minerals precipitate first.
Evaporite Sequence
- Carbonates (calcite and dolomite) are deposited first, usually when the original water volume is reduced by about 50%.
- Calcium sulfates (gypsum and anhydrite) precipitate next, when about 15% of the original water remains.
- Halite begins to form when the water volume is reduced to about 10%.
- The final, most soluble salts, including potassium and magnesium chlorides, precipitate last, often at less than 5% of the original water volume remaining.
Physical and Structural Characteristics
The physical characteristics of halite are directly related to its cubic crystal structure. This arrangement results in perfect cubic cleavage, causing the mineral to break cleanly into cubic fragments. Halite possesses a relatively low hardness, scoring only 2.0 to 2.5 on the Mohs scale.
While pure halite is colorless or white, the presence of impurities or crystal lattice defects causes variations in color. Iron-bearing clay minerals can tint the salt red or orange, while deep blue and purple hues are often the result of structural anomalies within the crystal itself. The density of halite is low compared to most surrounding rock types, typically around 2.17 grams per cubic centimeter.
Salt deposits are found in two primary large-scale structural forms: bedded deposits and diapirs. Bedded deposits are the simplest form, consisting of nearly horizontal or slightly inclined layers of rock salt interbedded with other sedimentary rocks like shale and anhydrite. These layers can extend over vast areas and represent the initial, undisturbed precipitation of salt.
Diapirs, or salt domes, represent the extreme structural deformation of these beds. Because halite is highly plastic and has a low density, the weight of overlying, denser sediments causes the salt to flow and migrate upward. This buoyant flow creates large, vertical, cylindrical or mushroom-shaped masses that pierce through the layers above them. Salt domes can be massive, reaching several kilometers in both diameter and vertical extent.